R.f.sputter plating method and apparatus employing control of ion and electron bombardment of the plating

ABSTRACT

AN R.F. SPUTTER PLATING METHOD AND APPARATUS IS DISCLOSED. THE PLATING APPARATUS INCLUDES A PARTIALLY EVACUATED DEPOSITION CHAMBER CONTAINING A PAIR OF ELECTRODES EXCITED WITH RADIO FREQUENCY ENERGY TO PRODUCE A RADIO FREQUENCY PLASMA DISCHARGE IN THE REGION THEREBETWEEN. THE R.F. DISCHARGE DEFINING ELECTRODES ARE OPERATED AT A FLOATING POTENTIAL SUCH THAT, BY THE NATURE OF THE PLASMA DISCHARGE, RELATIVELY HIGH NEGATIVE SELF-BIAS POTENTIAL IS ESTABLISHED ON THE R.F. ELECTRODES. A TARGET MATERIAL IS PLACED ON THE R.F. ELECTRODES TO BE SPUTTERED BY ION BOMBARDMENT, SUCH IONS EMANATING FROM THE PLASMA DISCHARGE. A COLLECTOR ELECTRODE IS DISPOSED TO RECEIVE THE SPUTTERED TARGET MATERIAL TO PLATE THE COLLECTOR ELECTRODE OR ITEMS CARRIED THEREON. A PAIR OF TRANSPARENT GRID STRUCTURES ARE DISPOSED INTERMEDIATE THE COLLECTOR ELECTRODE AND THE PLASMA DISCHARGE REGION. A FIRST ONE OF THE GRID STRUCTURES, ADJACENT THE PLASMA DISCHARGE, IS OPERATED AT GROUND POTENTIAL TO PREVENT PERTURBATION OF THE DISCHARGE, WHEREAS THE SECOND GRID STRUCTURE DISPOSED INTERMEDIATE THE FIRST GRID STRUCTURE AND THE COLLECTOR STRUCTURE IS OPERATED AT A POTENTIAL POSITIVE WITH RESPECT TO THE DISCHARGE FOR REPELLING POSITIVE IONS. THE COLLECTOR STRUCTURE IS OPERATED AT A POTENTIAL NEGATIVE WITH RESPECT TO THE SELF-BIAS POTENTIAL ON THE TARGET ELECTRODES TO REPEL ELECTRONS EMANATING FROM THE DISCHARGE REGION, WHEREBY ION AND ELECTRON BOMBARDMENT OF THE ITEMS BEING PLATED IS CONTROLLED.

Feb. 9, 1971 L JR v 3,562,142

APPARATUS R.F. SPUT PLAT METHOD AND LOY CONTROL EMF OF AND C'IRONBOMBARDMENT OF THE FLA G Filed Oct. 30, 1968 2 INVENTOR.

BY LAWRENCE T. LAMQNTJR ATTORNEY United States Patent 3,562,142 R.F.SPUTTER PLATING METHOD AND APPARA- TUS EMPLOYIN G CONTROL OF ION ANDELEC- TRON BOMBARDMENT OF THE PLATING Lawrence T. Lamont, Jr., PaloAlto, Calif., assignor to Varian Associates, Palo Alto, Calif., acorporation of California Filed Oct. 30, 1968, Ser. No. 771,797 Int. Cl.C23c 15/00 US. Cl. 204298 3 Claims ABSTRACT OF THE DISCLOSURE An R.F.sputter plating method and apparatus is disclosed. The plating apparatusincludes a partially evacuated deposition chamber containing a pair ofelectrodes excited with radio frequency energy to produce a radiofrequency plasma discharge in the region therebetween. The RF. dischargedefining electrodes are operated at a floating potential such that, bythe nature of the plasma discharge, a relatively high negative self-biaspotential is established on the RP. electrodes. A target material isplaced on the RP. electrodes to be sputtered by ion bombardment, suchions emanating from the plasma discharge. A collector electrode isdisposed to receive the sputtered target material to plate the collectorelectrode or items carried thereon. A pair of transparent gridstructures are disposed intermediate the collector electrode and theplasma discharge region. A first one of the grid structures, adjacentthe plasma discharge, is operated at ground potential to preventperturbation of the discharge, whereas the second grid structuredisposed intermediate the first grid structure and the collectorstructure is operated at a potential positive with respect to thedischarge for repelling positive ions. The collector structure isoperated at a potential negative with respect to the self-bias potentialon the target electrodes to repel electrons emanating from the dischargeregion, whereby ion and electron bombardment of the items being platedis controlled.

DESCRIPTION OF THE PRIOR ART Heretofore, radio frequency sputter platingapparatus has been employed for plating thin films of material ontodevices to be plated. The plating apparatus is typically characterizedby provision of a pair of R.F. electrodes in a partially evacuatedchamber. The electrodes are excited with radio frequency energy atapproximately 13.5 mhz. such electrodes being operated at a floatingpotential, whereby they self-bias themselves with respect to thepotential of the plasma discharge such that they are bombarded by ionsoriginating in the discharge to produce sputtering of the targetmaterial carried on the R.F. electrode. The target material is sputteredthrough the discharge onto a collector structure operated at groundpotential such structure also defining one side edge of the plasmadischarge. Such an RF. sputtering apparatus is described in the 14thAmerican Vacuum Society Vacuum Symposium Abstracts published by Herbickand Held Printing Company, Pittsburgh, Pa., 1967, authors, C. F. Lothropand L. F. Herte.

The problem with the prior art arrangement is that the collectorelectrode structure was operated at ground potential and was disposedimmediately adjacent and defined one side edge of the plasma discharge.As a consequence, high energy secondary electrons produced by ionbombardment of the RF. target electrodes were caused to bombard thecollector structure and the items being plated thereon. Bombardmentduring the initial stages of film growth greatly improves the adhesionof the film. It

'ice

is believed that this is due to the creation of nucleation sites on thesubstrate either by removal of surface contaminants or by the formationof surface defects. However, continued bombardment during the entirestage of building up the films above a few hundred Angstroms causing thefilm temperature to be elevated considerably above that of thesubstrate. This results in a large amount of stress in the surface filmwhich can result in poor adhesion or even flaking. In the case ofcopper, an additional effect is to reduce the reflectivity of thesurface. In the case of depositing aluminum oxide upon a glasssubstrate, bombardment of the substrate has a deleterious eflect uponthe glass substrate thereby changing its composition. Therefore, it isdesirable to control the electron and ion bombardment of the collectorstructure and the items being plated thereon at least after the initialstages of the plating cycle.

Control grids have been employed in DC. plasma discharge sputter platingapparatus such as that described in US. Pat. 3,361,659. However, thecharacter of that type of discharge is quite different from thatobtained in a radio frequency discharge as emloyed in the presentinvention. The mode of operation of the grids in a DC. discharge systemis nonanalogous to the mode of operation of such grids in an RF.discharge system.

SUMMARY OF THE INVENTION The principal object of the present inventionis the provision, in an R. F. sputter plating method and apparatus, ofcontrol of ion and electron bombardment of the plating.

One feature of the present invention is the provision, in an RF. sputterplating apparatus, of a structure disposed intermediate the plasmadischarge region and the collector electrode structure such gridstructure being operative with the collector potential for controllingbombardment of the collector structure by ions and elec trons passingout of the plasma discharge region by application of suitable potentialsto said grid structure and collector structure, whereby the deleteriouseffects of ion and electron bombardment of the deposited materials areavoided in use.

Another feature of the present invention is the same as the precedingfeature wherein a first grid is disposed adjacent the plasma dischargeand is operated at essentially ground potential to define one side ofthe plasma discharge without substantially perturbing the discharge.

Another feature of the present inevntion is the same as any one or moreof the preceeding features wherein a second grid is disposedintermediate a first grid structure and the collector structure, suchsecond grid being operated at a positive potential relative to thepotential of the plasma to reflect positive ions emanating from theplasma discharge, whereby ion bombardment of the plating is controlled.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the collector electrode structure isoperated at a negative potential relative to ground and preferably alsonegative with respect to the self-bias potential on the RP. electrodesfor reflecting electrons that have passed from the discharge through thefirst and second grids, whereby electron bombardment of the sputteredmaterial deposited on the collector structure is controlled.

Another feature of the present invention is a method for RF. sputterplating wherein the ion and electron bombardment of the collectedsputtered material is controlled by producing suitable barrierpotentials in the path of the sputtered target material between theradio frequency plasma discharge zone and the collecting structure forrepelling both positive ions and the electrons emanating from thedischarge.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram ofa radio frequency sputter apparatus of the present invention,

FIG. 2 is an enlarged cross-sectional view of a portion of the structureof FIG. 1 delineated by lines 2-2, and

FIG. 3 is a plot of potential versus distance taken along line 33 ofFIG. 2 depicting the potential barriers for reflecting ions andelectrons back to the plasma zone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown in schematic line diagram form, a radio frequency sputterplating apparatus 1 incorporating features of the present invention.This apparatus is of the general type descripe in the aforementionedarticle and has an inverted V-shaped configuration for the sputterelectrodes. However, the electrodes need not be of the inverted Vconfiguration but may, alternatively, be parallel to each other anddisposed parallel over the substarte member. Since these R.F. sputterplating apparatuses are commercially available and are described in thepublished literature they will not be described in detail in the presentdisclosure. Only the additional features of the control devices forcontrolling ion and electron bombardment of the plated surfaces will bedescribed in detail.

Briefly, the R.F. sputter plating apparatus 1 includes a bell jar 2forming an evacuable chamber in gas commnication with a vacuum system 3utilized for evacuating the bell jar to a suitable operating pressure,such as 2X10 torr. A suitable ionizable gas, such as argon, is utilizedas the fill for the bell jar such that an intense plasma discharge maybe obtained. A pair of R.F. electrodes 4 are supported within the belljar 2 and are preferably arranged in an inverted V-shaped configuration.Suitable R.F. electrodes 4 are made of, for example, copper. Theelectrodes 4 are connected across the secondary of a radio frequencytransformer 5, the primary of which is excited from a radio frequencygenerator 6 at a suitable radio frequency, such as 13.5 mHz. Thetransformer is an isolation transformer permitting a DC. potential to beestablished on the secondary to be independent of the DC.

potential on the primary circuit. In other words, the secration whereelectrodes 4 are parallel to each other and parallel to the surface of acollector electrode structure 7 which is disposed facing the electrodes4.

The collector electrode structure 7 is preferably water cooled in theconventional manner and is adapted to support, on its surface facing theelectrodes 4, the items or devices 8 which it is desired to plate. Atarget material 9 which it is desired to sputter, such as copper, gold,alumina, or the like, is carried from the inner faces of the R.F.electrodes 4. A variable capacitor 11 is connected across the secondarywinding of the transformer 5 for impedance matching the impedance of thesource 6 to the impedance of the plasma discharge established in theregion between the R.F. electrodes 4. A typical value for thecapacitance of capacitor 11 is from 17 to 72 picofarads. A pair of gridelectrode structures 12 and 13 are disposed over the face of thecollector electrode 7 and are supported therefrom in the manner asdepicted in FIG. 2.

Referring now to FIGS. 1 and 2, it is seen that grids 12 and 13, as of100 mesh grid, i.e., 100 meshes to the inch and made of 1 mil diameterwire, are supported in suitable frame structures from the collectorstructure 7 via the intermediary of sputter shielded insulatorassemblies 15 and 14, respectively. The first or inside grid 12 ispreferably operated at approximately ground potential, whereas thesecond grid 13 is preferably operated at a relatively low positivepotential, as of +200 volts, as derived from a power supply 16 viapotentiometer pick-off 17. The collector electrode 7 is preferablyoperated at a very substantial neagtive potential as of -2500 volts, asderived from power supply 16 via potentiometer pick-off 18. Grids 12 and13 preferably have as high a transparency for sputtered material aspossible and the aforementioned grids having 100 mesh and 1 mil wireprovide approximately transmission such that material sputtered fromtargets 9 passes through the grids 12 and 13 and is deposited upon thedevices 8 to be plated.

In opeartion, the radio frequency energy applied across the R.F.electrodes 4 produces a radio frequency plasma discharge in the regionbetween the plates 4 resulting in a plasma discharge zone 19. In theplasma discharge zone 19 there are approximately as many positive ionsas negative electrons such that the plasma zone is electrically neutraland at a relatively low potential with respect to ground, such as volts,depending upon the gas utilized in the discharge and the pressure of thedischarge.

The plasma discharge zone 19 extends almost to the surfaces of thetarget electrode portions 9 and to the grounded grid 12. Morespecifically, it is found that a dark space sheath of approximately 1.8centimeters in thickness is found immediately adjacent the interiorsurfaces of the target electrode portions 9, whereas the dark zonebetween the grid 12 and the plasma discharge zone 19 is approximately 2millimeters wide.

In the presence of the R.F. plasma discharge, the electrodes 4 and 9assume a negative DC. potential which is approximately equal to themagnitude of the average of the peak-to-peak amplitude of the appliedR.F. potential impressed across the plates 4 and 9. This relativelylarge D.C. self-bias potential which develops on electrodes 4 and 9 hasbeen attributed to the high mobility of the electrons relative to thatof the ions. Thus, during one cycle of operation, more electrons areattracted to any one of the plates 4 or 9 during the positive half cyclethan there are ions attracted during the negative half cycle, such thata negative charge builds up on each of the electrodes 4 including thetarget electrodes 9, if they are conductive and are conductivelyconnected to the electrodes 4. The end result is that both electrodes 4and the target electrodes 9, when they are conductive, acquire a largenegative DC. potential with respect to the plasma, such potentialtypically falling within the range of 1 kv. to 3 kv. and being afunction of the gas utilized to form the plasma discharge and thepressure of the discharge.

In operation, positive ions within the discharge zone 19 are acceleratedacross the dark sheath and bombard the target electrode 9 with an energycorresponding to the DO. bias, namely, typically about 2.3 kv. Thisresults in sputtering of the target material. The sputtered materialpasses through the discharge zone 19 and impinges upon the devices 8 tobe plated, as carried upon the collector structure 7. The gridstructures 12 and 13 are made sufficiently transparent such as not tointercept a substantial portion of the sputtered material. Typicaltransparencies for the grids 12 and 13 are approximately 80%.

When the positive ions bombard the target material 9, in addition todislodging target material, these particles also knock out secondaryelectrons in accordance with the secondary emission ratio of the targetmaterial. These secondary electrons see the R.F. voltage as superimposedupon the self-bias voltage and under certain phase angles of the R.F.voltage see very substantial accelerating voltages which propel thesecond electrons in a direction normally to the electrodes 4 and 9 backinto and through the plasma discharge zone 19. Since the discharge zoneis operating at relatively low pressure, for example, 1X10- torr to3X10- torr, the mean free path for the electrons is on the same order ofmagnitude as the typical dimensions of the evacuated system and,therefore, a large fraction of the propelled secondary electrons willpass through the discharge zone 19 and, in the absence of a barrierpotential on grids 12 and 13, through the grids 12 and 13 to strike theplating which is being deposited upon the devices 8 on the collectorelectrode 7. However, in use, as shown in FIG. 3, the potential appliedto the collector structure 7 and devices 8 is of a sufliciently negativepotential to repel or reflect the high energy electrons which havepassed through grids 12 and 13. By varying the potential picked oif bypick-off 18 on the power supply 16 the amount of electron bombardment ofthe devices 8 may be controlled.

In addition, positive ions are present in the plasma discharge zone 19.Since the plasma zone discharge zone 19 is operating at a potentialslightly positive with respect to ground a certain small fraction of thepositive ions within the discharge will be drawn out of the dischargeand propelled through the grounded grid 12 toward the collectorstructure 7. By applying a positive potential to the second grid 13relative to the plasma potential, typically +150 volts, such ions can bereflected by the grid 13 and returned to the plasma zone 19. Inoperation, a potential of approximately +200 volts on grid 13 is foundsufficient to reflect the positive ions back to the plasma dischargezone 19, when the zone 19 is operating at a potential of approximately+150 volts with respect to ground potential. A grounded shield 21surrounds the electrodes 4, 12, 13, and 7 for confining the plasmadischarge to the desired zone 19.

Thus, both positive ion and electron bombardment of the plated devices 8can be controlled during the plating process by selection of the correctpotentials applied to grids 12, 13 and collector 7. It is found thatsome bombardment of the surface being plated is desired during theinitial phases of the plating process. It is believed that thisbombardment of the surface serves to increase the adhesion between theplating and the surface being plated by removing surface contaminants orby the formation of surface defects serving as nucleation sites for theplating. However, in some cases, during the major portion of the platingtime and particularly during the final portion of the plating cycle, itis desired not to have substantial bombardment of the plating by eitherthe electrons or the ions as this bombardment can cause substantialoverheating of the film being plated and can also result in destructionof the substrate or surface on which the plating is being deposited. Theresult is that poor adhesion is obtained and even flaking is obtained insome cases. In the case of copper plating, an additional effect is toreduce the reflectivity of the surface.

In some cases, it is conceivable that it may be desirable to bombard thesurface being plated with the ions and not the electrons. In such acase, the potential applied to the ion reflecting grid 13 is selectedvia pick-off 17 such that the ions are not repelled by the second 'grid13. Once the ions pass through the grid 13 they are accelerated andpropelled against the surfaces being plated as 0 collector structure 7,the ion and electron bombardment of the surfaces being plated can bereadily controlled while the plasma zone remains substantiallyundisturbed due to the presence of the grounded grid structure 12.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A radio frequency sputtering apparatus comprising means defining apair of radio frequency electrode structures, means for impressing aradio frequency potential across said pair of radio frequency electrodesto produce a radio frequency plasma discharge, means forming a targetelectrode structure including material to be sputtered and disposed suchas to be bombarded by ions emanating within said plasma dischargeregion, means forming a collector electrode structure disposed toreceive target material sputtered from said target electrode structure,means forming a grid electrode structure hav ing first and secondportions disposed intermediate said plasma discharge region and saidcollector electrode structure and operative with said collectorstructure by application of suitable electrical potentials thereto forcontrolling bombardment of said collector electrode structure by chargedparticles passing out of said plasma discharge region, means forapplying substantially ground potential to said first portion of saidgrid structure, said first portion of said grid structure being disposedadjacent said plasma discharge region whereby said first portion of saidgrid structure defines one side of said plasma discharge region withoutsubstantially perturbing the discharge, said second portion of said gridstructure being insulated from said first portion and disposedintermediate said first grid portion and said collector structure, andmeans for applying a positive potential to said second grid portionrelative to said first portion to control passage of positive ionsemanating from said plasma discharge.

2. The apparatus of claim 1 including, means for applying a negativepotential to said collector structure relative to ground potential forcontrolling passage of electrons from said plasma discharge region tosaid collector electrode whereby electron bombardment of the sputteredmaterial deposited on said collector structure is controlled.

3. The apparatus of claim 2 wherein said means for applying a negativepotential applies a potential which is negative with respect to aself-bias DC. potential established on said pair of radio frequencyelectrodes.

References Cited UNITED STATES PATENTS 3,410,775 11/ 1968 Uranty 204-2983,361,659 1/1968 Bertelsen 204298 3,267,015 8/ 1966 Morley 204-298 OTHERREFERENCES Kloss, F., et al., Advances in R-F-Induced Plasma Sputtering,SCP and Solid State Tech., December 1967.

JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner

